IONIC BONDING - ALL YOU NEED TO KNOW ABOUT THE CHEMISTRY OF IONIC BONDING
In nature, achieving a filled (complete) valence energy level is a driving force of chemical reactions, because when that energy level is full, elements become stable, or “satisfied” — stable elements don’t lose, gain, or share electrons.
The noble gases — the VIIIA elements on the periodic table — are extremely nonreactive because their valence energy level (outermost energy level) is filled. However, the other elements in the A families on the periodic table do gain, lose, or share valence electrons to fill their valence energy level and become satisfied.
Because filling the valence energy level usually involves filling the outermost s and p orbitals, it’s sometimes called the octet rule — elements gain, lose, or share electrons to reach a full octet (eight valence electrons: two in the s orbital and six in the p orbital).
Gaining and losing electrons
When an atom gains or loses an electron, it develops a charge and becomes an ion. In general, the loss or gain of one, two, or sometimes even three electrons can occur, but an element doesn’t lose or gain more than three electrons.
Losing an electron to become a cation: Sodium Ions that have a positive charge due to the loss of electrons are called cations. In general, a cation is smaller than its corresponding atom. Why? The filled energy level determines the size of an atom or ion, and a cation gives up enough electrons to lose an entire energy level.
Consider sodium, an alkali metal and a member of the IA family on the periodic table. Sodium has 1 valence electron and 11 total electrons, because its atomic number is 11. It has an electron configuration of 1s22s22p63s1.
By the octet rule, sodium becomes stable when it has eight valence electrons. Two possibilities exist for sodium to become stable: It can gain seven more electrons to fill energy level 3, or it can lose the one 3s electron so that energy level 2 (which is already filled at eight electrons) becomes the valence energy level.
So to gain stability, sodium loses its 3s electron. At this point, it has 11 protons (11 positive charges) and 10 electrons (10 negative charges). The once-neutral sodium atom now has a single positive charge [11 (+) plus 10 (–) equals 1+]. It’s now an ion, an atom that has a charge due to the loss or gain of electrons. You can write an electron configuration for the sodium cation: Na+: 1s22s22p6
Note that if an ion simply has 1 unit of charge, positive or negative, you normally don’t write the 1; you just use the plus or minus symbol, with the 1 being understood.
Atoms that have matching electron configurations are isoelectronic with each other. The positively charged sodium ion (cation) has the same electron configuration as neon, so it’s isoelectronic with neon. So does sodium become neon by losing an electron? No. Sodium still has 11 protons, and the number of protons determines the identity of the element.
There’s a difference between the neutral sodium atom and the sodium cation: one electron. As a result, their chemical reactivities are different and their sizes are different. Because sodium loses an entire energy level to change from a neutral atom to a cation, the cation is smaller.
LEWIS THEORY - CHEMISTRY TUTORIALS
Something unique in the electron configurations of noble gas atoms accounts for their inertness, and atoms of other elements combine with one another to acquire electron configurations like those of noble gas atoms.
The theory that grew out of this model has been most closely associated with G. N. Lewis and is called the Lewis theory. Some fundamental ideas associated with Lewis s theory follow:
1. Electrons, especially those of the outermost (valence) electronic shell, play a fundamental role in chemical bonding.
2. In some cases, electrons are transferred from one atom to another. Positive and negative ions are formed and attract each other through electrostatic forces called ionic bonds.
3. In other cases, one or more pairs of electrons are shared between atoms. A bond formed by the sharing of electrons between atoms is called a covalent bond.
4. Electrons are transferred or shared in such a way that each atom acquires an especially stable electron configuration. Usually this is a noble gas configuration, one with eight outer-shell electrons, or an octet.
Lewis developed a special set of symbols for his theory. A Lewis symbol consists of a chemical symbol to represent the nucleus and core (inner-shell) electrons of an atom, together with dots placed around the symbol to represent the valence (outer-shell) electrons.
A Lewis structure is a combination of Lewis symbols that represents either the transfer or the sharing of electrons in a chemical bond.
In Lewis theory, we use square brackets to identify ions, as we did in equation (10.1). The charge on the ion is given as a superscript.
Lewis s work dealt mostly with covalent bonding, which we will emphasize throughout this chapter. However, Lewis s ideas also apply to ionic bonding, and we briefly describe this application next.
Subscribe to:
Comments (Atom)